Multicasting And Multicast Services Overview; Understanding How Igmp Snooping Works - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Understanding How Multicasting Works

Multicasting and Multicast Services Overview

IGMP snooping manages multicast traffic in switches by allowing directed switching of IP multicast
traffic. GMRP is protocol independent and can manage both IP multicast traffic and any Layer 2
multicast traffic.
Switches can use IGMP snooping or GMRP to configure switch ports dynamically so that IP multicast
traffic is forwarded only to those ports associated with IP multicast hosts. IGMP software components
run on both the Cisco router and the switch.
For more information on IP multicast and IGMP, refer to RFC 1112. GMRP is described in
Note
IEEE 802.1p.
You can statically configure multicast groups using the set cam static command. Multicast groups
learned through IGMP snooping are dynamic. If you specify group membership for a multicast group
address, your static setting supersedes any automatic manipulation by IGMP snooping or GMRP.
Multicast group membership lists can consist of both user-defined setting and setting learned through
IGMP snooping or GMRP.

Understanding How IGMP Snooping Works

You can run IGMP snooping on any Catalyst 6000 family supervisor engine model (Supervisor Engine 1,
Note
Supervisor Engine 1A, and Supervisor Engine 2). A PFC is not required to enable IGMP snooping. Cisco
Group Management Protocol (CGMP) is not supported on the Catalyst 6000 family switches, although
CGMP server is supported on the MSFC. To support CGMP client devices, configure the MSFC as a
CGMP server.
IGMP snooping manages multicast traffic at Layer 2 on the Catalyst 6000 family switches by allowing
directed switching of IP multicast traffic.
Switches can use IGMP snooping to configure Layer 2 interfaces dynamically so that IP multicast traffic
is forwarded only to those interfaces associated with IP multicast devices.
Catalyst 6000 switches can distinguish IGMP control traffic from multicast data traffic. When IGMP is
enabled on the switch, IGMP control traffic is redirected to the CPU for further processing. This process is
performed in hardware by specialized ASICs, which allow the switch to snoop IGMP control traffic with no
performance penalty.
The route processor periodically sends out general queries to all VLANs, and as multicast receivers
respond to the router's queries, the switch intercepts them. Only the first IGMP join (report) per VLAN
and per IP multicast group is forwarded to the router. Subsequent reports for the same VLAN and group are
suppressed. The switch processor creates one entry per VLAN in the Layer 2 forwarding table for each
MAC group from which it receives an IGMP join request. All hosts interested in this multicast traffic
send join requests and are added to the port list of this forwarding table entry.
If a port is disabled, it will be removed from all multicast group entries.
You cannot enable IGMP snooping on a switch if GMRP is already enabled on the switch.
Note
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
40-2
Chapter 40 Configuring Multicast Services
78-13315-02